This plastic sheet (usually, but not
always, green) is often used to inspect magnetic parts, especially
for the transitions between magnetic poles (north-south transitioning
to south-north, etc.). Unfortunately, a lack of understanding of the
way the sheet is constructed often leads to misinterpretation of the
results.

Microscopic flakes of nickel are first
coated with an oil, in which a plastic material has been dissolved.
The plastic then separates out, forming a skin around the oil drop.
The flake is then free to rotate within the shell of plastic, which
is invisibly small in diameter.

Many layers of these spheres (perhaps
30) are deposited onto a plastic sheet, which forms a support. The
support sheet may be on the order of 0.005 in thick, and the layers
of spheres may add on the order of 0.002 in to the total thickness.

When no magnetic field is present, the
flakes lie flat in the bottom of their spheres, reflecting upward the
color of the plastic sheet. When a magnetic field is present in the
plane of the sheet, the brightness is intensified.

On the other hand, if a magnetic field
is present which is normal to the sheet—that is, at approximately
right angles into or out of the sheet—the flakes stand on end,
aligning with the field. When this occurs, the light reflected off
them bounces back and forth until it is absorbed, in a manner similar
to the light in a metal tube, and no light is reflected (it is
black).

It can be seen, then, that the green
color means either that no field is present, or that there is a
field, in the plane of the sheet. For example, a region in which flux
is leaving the sheet at 45° from left to right as it rises will
appear to be black when viewed from the right side.

The same region viewed from the left
side, however, will appear to be green! In order to have a consistent
result from the indications of this material, it must be viewed from
directly overhead, not from an angle.

The nickel flakes saturate at a
relatively small field. This observer noted a change in color at
about 10 G and full transition to black at about 100 G for one type
of sheet.

The transition width for two
neodymium-iron magnets side by side in air may be from on the order
of +4000 G to −4000 G, but the part of this transition which is
indicated by the plastic sheet is much narrower—on the order of
1⁄40 as wide.

Based on the indications of the plastic
sheet, some have thought they were seeing a very narrow transition
between magnet poles, to a degree which is physically impossible.